This invention relates to an on-machine calender which can be connected to a paper machine or the like for finishing treatment of a fiber web, which calder includes a first hard roll and a second hard roll at a distance from the first one and in addition at least two elastic rolls mounted on bearings in movable supporting means in a way which permits the elastic rolls to be moved into a working position so as to define calendering nips with the hard rolls and, in addition, a number of paper guiding rolls for leading the web run through said calendering nips.
Furthermore the invention relates to a method in an on-machine finishing treatment of a web-like material which treatment takes place in a calendering apparatus in which a first and a second hard roll and at least two elastic rolls are employed, the elastic ones being arranged so as to define calendering nips with said hard rolls which nips all are substantially at a same horizontal level and in which calender the hard rolls are substantially at a same horizontal level and in which the elastic rolls are, with respect to each other, substantially at a same horizontal level which is above that of the hard rolls.
An important stage in finishing treatment of the paper is its calendering by which an influence exerted on the smoothness and gloss of the surface of the paper and the thickness and the density of the paper. Calendering takes place while passing the continuous paper web through particular pressing points, or nips, formed between coacting calender rolls.
Conventionally the calendering of paper is effected by means of a so-called machine calender which is directly associated with the paper machine said calendering treatment being complemented, if necessary, by a supercalendering treatment in a separate so-called supercalender which gives the paper more gloss.
The rolls used in calenders are either hard or elastic rolls. By hard rolls are meant in this context rolls having a shell material of e.g. chill-cast iron or steel and of which the surface acting on the paper is ground glossy. A conventional machine calender has hard rolls only and the nips between them are so-called hard nips.
Elastic rolls refer in the following to rolls in which the surface acting on the surface of the web is of resilient material. This kind of rolls which are used in supercalenders define together with the hard rolls so-called soft nips.
As material for the mantles of the elastic rolls paper sheets are commonly used which are cut into disks, assembled as courses on the roll shaft and thereafter pressed in a direction parallel with the shaft into a solid, compact and quite thick covering of the rolls. Nowadays various plastic-based coverings are also used which are commonly relatively thin due to which it is possible to equip the body of the roll with internal deflection compensation means without increasing the diameter of the roll to an excessive extent.
As is known in the prior art, machine-calendering can also be carried out with a single-nip calender, that is with a calender in which only one pair of rolls defines a nip, depending on the brand of the paper to be treated and the requirements therein imposed. In most cases, however, a machine-calender includes four to eight rolls which thus define three to seven hard nips. As a result of the machinecalendering process the possible thick areas in the web are smoothed out so that the web achieves the required thickness, or the so-called caliper.
Conventionally, by means of a supercalender provided with soft nips only, it is usually attempted to achieve equal gloss for both sides of the web. This requires at least two soft nips and in addition arranged usually in such a way that both sides of the web will be against the surface of a hard roll in an equal number of nips which surface primarily gives gloss for the web. A separate supercalender may include as many as ten pairs of nips. As a result of supercalendering the web usually acquires even density and smoothness.
To boost the production of a paper machine it has been found necessary to attempt to accomplish such an immediately to a paper machine connected calender unit which combines the functions of both a machine-calender and a supercalender. Such calenders have been disclosed e.g. in U.S. Pat. Nos. 4,128,053; 4,332,191; and 4,375,118.
Although machine-calenders and supercalenders usually include eight to ten nips it has been found that even two soft nips alone can give the paper produced such a gloss and smoothness and/or additional properties which are sufficient for most purposes. This results from the fact that the paper coming from the paper machine to calendering treatment is in respect of its formation and fibre distribution usually considerably more even and can be thus more easily calendered than e.g. paper produced according to the technology in use of e.g. about twenty years ago due to the today's web-formation techniques and the control systems therein involved. The development on one hand in the materials for elastic coverings and on the other hand in the properties of the paper to be glazed which can be influenced by applying a thin layer of a suitable paste on the surface of the paper, or coating the semi-finished web e.g. at some stage of the drying in a paper machine, makes it possible to use calender structures which differ from and are even more simple than the previous ones.
The most generally accepted comprehension of the influence of the supercalendering on the web to be processed is that a higher gloss is produced on the surface facing the hard roll in the nip. There are, however, several factors which together affect the gloss and smoothness such as nip load, the possible difference in the rotation speeds of the elastic roll and the hard roll, and as an important factor, the material of the elastic covering. Development work is continuing, especially regarding the last-mentioned factor, and covering materials new and/or under development may give reason to revise theories about calendering.